Transparent electrode, manufacturing method thereof and electronic device employing the transparent electrode

ABSTRACT

The present embodiments provide a transparent electrode having a laminate structure of:
         a first metal oxide layer having an amorphous structure and electroconductivity,   a metal layer made of a metallic material containing silver or copper,   a second metal oxide layer having an amorphous structure and electroconductivity, and   a third metal oxide layer having an amorphous structure and continuity, stacked in this order.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2017-142004, filed on Jul. 21,2017, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments of the present disclosure relate to a transparent electrode,a manufacturing method, and an electronic device employing thetransparent electrode.

BACKGROUND

According to recent increasing of energy consumption, demand foralternative energy replacing fossil fuels has been rising as measures tocope with global warming. As a source of the alternative energy, solarcells have been attracted the attention and hence progressivelydeveloped. They are studied for various practical applications. From apractical viewpoint, it is particularly important to improve theirflexibility and durability so that they can be installed on variousplaces. However, single crystal silicon solar cells, which are the mostbasic solar cells, are expensive and it is difficult to produce flexibleones. On the other hand, organic solar cells and organic-inorganichybrid solar cells, in which many researchers have been latelyinterested, have room for improvement in durability.

In addition to the above solar cells, other photoelectric conversiondevices, such as organic EL devices and photosensors, have been alsoresearched with the aim of improving their flexibility and durability.Those devices usually comprise indium-doped tin oxide (hereinafter,often referred to as “ITO”) in the form of films serving as transparentelectrodes. The ITO films are normally formed by sputtering or the like.In order that the ITO films can have high electroconductivity, it isnecessary to carry out the sputtering procedure at a high temperatureand to anneal thereafter the formed films at a high temperature.However, it is often the case that organic materials cannot be subjectedto those procedures at a high temperature.

Meanwhile, transparent electrodes in electronic devices can be made ofITO/Ag/ITO composite material, which has low electronic resistance andhigh transparency. There is an examination case in which thecomposite-material electrode is adopted in a device comprising aPEDOT/PSS layer. Even so, however, amorphous ITO (hereinafter, oftenreferred to as “a-ITO”) and silver are liable to be deteriorated byacids or halogens diffusing from the outside or from other layers.Further, silver itself tends to easily migrate into other layers. Ifmigrating into other layers, silver reacts with water or the like toproduce silver oxide or the like, so that it may impair the transparentelectrode. Not only that, but also the migrating silver may reach intothe active area of the electronic device and consequently may lower theperformance of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing the structure of atransparent electrode according to the embodiment.

FIG. 2 is a schematic sectional view showing the structure of aphotoelectric conversion device (solar cell) according to theembodiment.

FIG. 3 is a schematic sectional view showing the structure of aphotoelectric conversion device (organic EL device) according to theembodiment.

FIG. 4 is a schematic sectional view showing the structure of a liquidcrystal device according to the embodiment.

FIGS. 5A-5E schematically illustrate a process for manufacturing adevice according to the embodiment.

FIG. 6 shows a sectional SEM image of the transparent electrode inExample 1.

FIG. 7 is a schematic sectional view showing the structure of thetransparent electrode in Example 7.

FIG. 8 is a schematic sectional view showing the structure of the solarcell in Example 8.

FIG. 9 is a schematic sectional view showing the structure of theorganic EL device in Example 9.

FIG. 10 is a schematic sectional view showing the structure of theliquid crystal cell in Example 11.

DETAILED DESCRIPTION

Embodiments will now be explained with reference to the accompanyingdrawings.

The transparent electrode according to the embodiment has a laminatestructure of

a first metal oxide layer having an amorphous structure andelectroconductivity,

a metal layer which comprises a metallic material containing silver orcopper,

a second metal oxide layer having an amorphous structure andelectroconductivity, and

a third metal oxide layer having an amorphous structure and continuity,stacked in this order.

Further, the electronic device according to the embodiment comprises theabove transparent electrode and a counter electrode.

Furthermore, the method according to the embodiment for manufacturing atransparent electrode, comprises the steps of:

-   (I) preparing a laminate structure of

a first metal oxide layer having an amorphous structure andelectroconductivity,

a metal layer which comprises a metallic material containing silver orcopper, and

a second metal oxide layer having an amorphous structure andelectroconductivity; and thereafter

-   (II) coating the surface of said second metal oxide layer with a    metal alkoxide solution, then drying and heating the applied    solution at 150° C. or less in a water-containing atmosphere, to    form a third metal oxide layer.

The embodiments are described below in detail.

Embodiment 1

First, the structure of the transparent electrode according to the firstembodiment is explained with reference to FIG. 1.

FIG. 1 schematically shows the structure of a transparent electrode 10according to the embodiment. The transparent electrode comprises a firstmetal oxide layer 12/a metal layer 13/a second metal oxide layer 14/athird metal oxide layer 15, stacked in this order. The transparentelectrode may be generally formed on a substrate 11. The substrate 11 isnot necessarily indispensable, but makes it easy to carry out proceduresfor producing the transparent electrode. Accordingly, the substrate maybe used only in the production process and hence may be removed afterthe laminate structure of the electrode is formed.

There are no particular restrictions on the material of the substrate.However, if the transparent electrode of the embodiment is employed incombination of the substrate, the substrate preferably has transparencyand flexibility. In view of that, preferred are resin materials, suchas, polyethylene terephthalate (hereinafter, referred to as “PET”) andpolyethylene naphthalate (hereinafter, referred to as “PEN”). Further,the substrate is preferably subjected to planarizing treatment.

The first metal oxide layer 12 and the second one 14 contain metaloxides, which can be freely selected from generally knownelectroconductive metal oxides. Examples thereof include: indium-dopedtin oxide (ITO), fluorine-doped tin oxide (FTO), and aluminum-doped zincoxide AZO). Other metal oxides may be adopted as long as they satisfythe requirements of transparency and electroconductivity. However, oneor both of the first and second metal oxide layers preferably contain ametal oxide selected from the group consisting of ITO, AZO and FTO. Thefirst and second metal oxide layers 12 and 14 may contain a substanceother than metal oxides, but preferably consist only of metal oxides ormetal composite oxides. Among the above, ITO is preferred because thezeta potential thereof is so close to 0 in the neutral pH range as toreduce the interaction with cations or anions. The first and secondmetal oxide layers have amorphous structures, from which homogeneous andeven layers are easily formed. Further, those metal oxide layerspreferably have continuity. The continuity of the metal oxide layer isexplained later in detail.

The first and second metal oxide layers are preferably thin enough toimprove the transparency and to shorten the production process. On theother hand, however, they are preferably thick enough to reduce theresistance. In view of those, the first and second metal oxide layerspreferably have thicknesses of preferably 30 to 200 nm, more preferably35 to 100 nm, further preferably 40 to 70 nm. The metal oxide layershaving thicknesses of 40 to 70 nm are particularly preferred becausethey have a good balance among transparency, electroconductivity andfilm-forming time. If the thickness is less than 30 nm, theelectroconductivity tends to be lowered. If it is more than 200 nm, thelight transmittance and the film-forming time are liable to be impairedand prolonged, respectively. It is necessary to pay attention to those.

The first and second metal oxide layers can be formed, for example, bysputtering at a low temperature. Specifically, the sputtering procedureis preferably carried out at a temperature of 150° C. or less. Thesputtering procedure at such a low temperature makes it possible to forma metal oxide film having an amorphous structure. The formed amorphousmetal oxide film can be annealed to partly crystalize and thereby toproduce a crystalline-amorphous composite film. The annealing procedureis preferably carried out in a high-temperature atmosphere and/or bymeans of laser annealing technique. However, the annealing procedure ispreferably carried out at a temperature of 150° C. or less so that themetal oxide film may not excessively crystalized.

The metal layer placed between the first and second metal oxide layerscomprises a metallic material containing silver or copper. Here, the“metallic material containing silver or copper” is a general term whichincludes silver metal, copper metal, silver-containing alloys,copper-containing alloys, and alloys containing both. While silver hassuperior electroconductivity but tends to migrate easily, copper hasinferior electroconductivity but higher migration resistance than silverand is less expensive than silver. The metal layer preferably consistsonly of a metallic material containing silver or copper.

The metal layer is preferably thin enough to improve the transparencybut preferably thick enough to reduce the resistance. Specifically, thethickness thereof is preferably 4 to 20 nm, more preferably 5 to 15 nm,further preferably 6 to 10 nm. The metal layer 13 can be formed, forexample, by sputtering or vapor deposition, and is preferably formed bysputtering. If being thinner than 4 nm, the metal layer tends to bediscontinuous. On the other hand, if it is thicker than 20 nm, thetransparency tends to be lowered. It is necessary to pay attention tothose.

The third metal oxide layer is placed outside of the second one. Thismeans that the metal layer, the second metal oxide layer and the thirdmetal oxide layer are stacked in this order. Since they are stacked inthat order, the third metal oxide layer can prevent silver or coppercontained in the metal layer from migrating to the outside of the metallayer. Further, acids or halogens entering from the outside of theelectrode can be prevented from diffusing into the metal layer.

In order to fully fulfill that function, the third metal oxide layer hascontinuity. The “continuity” means that crystalline parts existing inthe layer do not penetrate in the thickness direction. In theembodiment, the third metal oxide layer necessarily has an amorphousstructure. If the whole metal oxide layer consists only of the amorphousstructure and hence does not comprise any crystalline structure, thelayer has continuity. However, if the metal oxide layer partly containscrystalline parts, there are “gaps” among crystallites containedtherein. Accordingly, if the layer contains many crystallites, the gapsare aligned in series in the thickness direction to form a penetratingcavity. The penetrating cavity may serve as a path through which silver,acids or halogens can diffuse, and therefore the layer needs to have astructure in which the above path is not formed for the purpose ofpreventing the diffusion of silver, acids or halogens.

Even if the third metal oxide layer may contain crystalline parts, itcan be said that “the layer has continuity” provided that there are onlya small number of or, particularly, no penetrating cavities formed bythe crystalline parts aligned in series in the thickness direction. Themetal oxide in amorphous state and that in crystalline state aredifferent in reflection and transmission of electron beams and henceshow distinguishably different contrasts in an electron microscopicimage. It is, therefore, possible to electron-microscopically observe apenetrating cavity formed by the crystalline metal oxide. Thus, whetherthe metal oxide layer 15 has continuity or not can be judged byobserving a section of the third metal oxide layer with an electronmicroscope.

Specifically, the continuity can be evaluated in the following manner. Asection of the third metal oxide layer is observed at a magnification of100000 times. The objective section is divided into ten 1000 nm-widthobserved areas, which are individually evaluated. If the penetratingcavities are visually confirmed only in two or less of the ten observedareas, the metal oxide layer is considered to have continuity in theembodiment. The penetrating cavities are preferably found in zero of theten areas. In other words, it is preferred that no penetrating cavitiesbe found in any of the ten areas.

The third metal oxide layer is preferably thick enough to enhance theeffect of preventing migration of metals and the like, but preferablythin enough to reduce the resistance thereof so that electric charge canmove easily. In view of those, the third metal oxide layer has athickness of preferably 5 to 50 nm, more preferably 10 to 40 nm, furtherpreferably 15 to 30 nm.

The second metal oxide layer 14 is preferably thicker than the third one15. That structure makes it easy to achieve a balance between theelectroconductivity and the function of preventing migration of metals.

The third metal oxide layer can be formed by various methods, such as,sputtering and sol-gel processes. However, it is preferred to form thelayer by the steps of applying an alcohol solution of metal alkoxide andthen heating the applied solution in a water-containing atmosphere. Thatprocess makes it possible to form a thin, large-area and even amorphousfilm.

There is an interface between the third metal oxide layer and the secondone. The third metal oxide layer is generally made of materials properlyselected according to the aimed use of the transparent electrode, andhence the third and second metal oxide layers are often different inmaterials from each other. In that case, the interface is inevitablyformed therebetween. However, according to necessity, they may comprisethe same materials. In that situation, another layer may be providedtherebetween or otherwise they may be formed in such different mannersas to have different properties, so that the interface can be formedtherebetween. If they are formed from the same materials in the samemanner, the interface is not formed and accordingly they are regarded asa single metal oxide layer.

The materials of the third metal oxide layer can be properly selectedaccording to the aimed use. For example, they can be selected from thegroup consisting of titanium oxide, tin oxide, zinc oxide, and zirconiumoxide. Those substances easily serve as n-type semiconductors, andtransparent electrodes comprising them are preferably employed ascathodes of photoelectric conversion devices. Among them, preferred aretitanium oxide and zirconium oxide because stable films thereof can beeasily formed and also because the zeta potentials thereof are so closeto 0 in the neutral pH range as to reduce the interaction with cationsor anions. Further, in consideration of raw material supply, titaniumoxide is particularly preferred.

Further, the materials of the third metal oxide layer may be alsoselected from the group consisting of nickel oxide, molybdenum oxide,iron oxide and copper oxide. Those substances easily serve as p-typesemiconductors, and transparent electrodes comprising them arepreferably employed as anodes of photoelectric conversion devices.

Among them, nickel oxide is preferred because a stable film thereof canbe easily formed and also because the zeta potential thereof is so closeto 0 in the neutral pH range as to reduce the interaction with cationsor anions.

Furthermore, the materials of the third metal oxide layer may be stillalso selected from the group consisting of silicon oxide and aluminumoxide. Those substances easily form high resistance films, andtransparent electrodes comprising them are preferably employed involtage-drive type elements and touch panels. Among them, aluminum oxideis preferred because a stable film thereof can be easily formed and alsobecause the zeta potential thereof is so close to 0 in the neutral pHrange as to reduce the interaction with cations or anions.

The third metal oxide layer can be doped with other ions having avalence different from that of the metal constituting the metal oxide inthe layer, so as to improve the electroconductivity of the layer or tochange the energy level thereof.

For example, it is preferred to dope a titanium oxide layer withpentavalent elements. Examples of the pentavalent elements include:nitrogen, phosphorus, niobium, vanadium, tantalum, arsenic, antimony andthallium. If doped with those elements, the metal oxide layer tends tobehave as an n-type semiconductor layer.

It is also preferred to dope a molybdenum oxide layer with pentavalentelements. Examples of the pentavalent elements include: nitrogen,phosphorus, niobium, vanadium, tantalum, arsenic, antimony and thallium.If doped with those elements, the metal oxide layer tends to behave as ap-type semiconductor layer.

The third metal oxide layer preferably contains oxygen deficiencies,which can bring the layer more amorphous to change the energy level. Theoxygen deficiencies can be measured by X-ray photoelectron spectroscopy(XPS).

For example, titanium oxide is preferably represented by TiO_(x) (inwhich x is less than 2). The value x less than 2 indicates that theoxide contains oxygen deficiencies, which bring the layer moreamorphous. It can be judge by XPS whether or not the titanium oxidecontains oxygen deficiencies. If there are oxygen deficiencies, theresultant XPS spectrum shows that the peak corresponding to Ti has aswollen tail on the low-energy side.

The third metal oxide layer may contain carbon or nitrogen, which canbring the layer more amorphous. Those atoms can be measured by XPS.

Further, the third metal oxide layer may contain alkoxy-metal bonds. Forexample, if a metal alkoxide is used in the process for producing thethird metal oxide layer, alkoxy-metal bonds are often introduced intothe layer. In that case, some alkoxy-metal bonds in the metal alkoxideare decomposed by oxidation reactions or the like but the others areleft. If remaining in the layer, the alkoxy-metal bonds bring the layermore amorphous and enhance the ability to function as an ion barrier.The alkoxy-metal bonds can be measured by IR or XPS. For example, iftitanium oxide contains the alkoxy-metal bonds, the resultant XPSspectrum shows that the peak corresponding to oxygen has a swollen tailon the high-energy side.

It is also possible to form a graphene layer in direct contact with thethird metal oxide layer. The graphene layer may be positioned on themetal-layer side, on the side opposite to the metal layer or on eachside of the third metal oxide layer.

The graphene layer preferably has a laminate structure of 1 or more but4 or less graphene monomolecular films (hereinafter, often referred toas “monolayer graphenes”) on average. The graphene layer is preferablyeither unsubstituted or doped with nitrogen. The nitrogen-doped graphenelayer is preferably used in a cathode. The doping amount (N/C atomicratio) can be measured by XPS, and is preferably 0.1 to 30 atom %, morepreferably 1 to 10 atom %. The graphene layer has such high shieldingability as to prevent acids and halogens from diffusing and thereby toavoid deterioration of the metal oxide layers and the metal layer. As aresult, the impurities coming from the outside can be prevented by thegraphene layer from entering the photoelectric conversion layer.Further, since containing nitrogen atoms, the nitrogen-doped graphenelayer has high acid-trapping ability and hence has higher shieldingability.

The transparent electrode according to the embodiment may furthercomprise a UV-cut layer and/or a gas-barrier layer. Examples ofultraviolet absorbers usable in the UV-cut layer include: benzophenones,such as, 2-hydroxy-4-methoxy-benzophenone,2,2-dihydroxy-4-methoxybenzophenone,2-hydroxy-4-methoxy-2-carboxybenzophenone, and2-hydroxy-4-n-octoxybenzophenone; benzotriazoles, such as,2-(2-hydrxy-3,5-ditertiarybutylphenyl)benzotriazole,2-(2-hydrxy-5-methylphenyl)benzotriazole, and2-(2-hydrxy-5-tertiaryphenyl)benzotriazole; and salicylic esters, suchas, phenyl salicylate and p-octylphenyl salicylate. The absorbers arepreferably capable of cutting UV light in the wavelength range of 400 nmor shorter.

The gas-barrier layer blocks preferably water vapor and oxygen,particularly preferably water vapor. That layer is preferably, forexample, a film of inorganic substance such as SiN, SiO₂, SiC,SiO_(x)N_(y), TiO₂ or Al₂O₃. Further, it may be a super-thin glasssheet. There are no particular restrictions on the thickness of thegas-barrier layer. The gas-barrier layer is preferably thick enough toserve fully as a barrier against gases.

On the other hand, however, the layer is preferably thin enough toensure flexibility or bendability. In view of those, the gas-barrierlayer has a thickness of preferably 0.01 to 3000 μm, more preferably 0.1to 100 μm. The gas-barrier layer has a water-vapor permeability(moisture permeability) of preferably 10² to 10⁻⁶ g/m²·d, morepreferably 10¹ to 10⁻⁵ g/m²·d, further preferably 10⁰ to 10⁻⁴ g/m²·d.The moisture permeability can be measured according to ES 20208 and thelike. The gas-barrier layer is preferably formed by dry processes.Examples of the dry processes for forming the gas-barrier layer include:vacuum deposition processes, such as, resistance heating deposition,electron beam deposition, induction heating deposition, and plasma orion assisted deposition; sputtering processes, such as, reactivesputtering, ion beam sputtering, ECR (electron cyclotron resonance)sputtering; PVD (physical vapor deposition) processes, such as, ionplating; and CVD (chemical vapor deposition) processes employing heat,light or plasma. Among them, preferred are vacuum deposition processes,in which a film of the layer is formed by deposition in vacuum.

If the transparent electrode according to the embodiment comprises asubstrate, the substrate is selected according to the aimed use. Forexample, transparent substrates can be made of inorganic substances,such as, glass; and organic substances, such as, PET. PEN, polycarbonateand PMMA. Particularly preferred is a substrate made of flexible organicsubstances so that the resultant transparent electrode of the embodimentcan have excellent flexibility.

Embodiment 2-1

The structure of a photoelectric conversion device according to thesecond embodiment is explained below with reference to FIG. 2. FIG. 2schematically shows the structure of a solar cell (photoelectricconversion device) 20 according to the embodiment. The solar cell 20 hasa function of converting photoenergy hv of incident sunlight or the likeinto electric power. The solar cell 20 comprises a transparent electrode21, a photoelectric conversion layer 22 formed on a surface of thetransparent electrode 21, and a counter electrode 23 formed on thesurface of the photoelectric conversion layer 22 on the side opposite tothe transparent electrode 21.

The transparent electrode 21 is the same as that described inEmbodiment 1. Specifically, it has a laminate structure of: a firstmetal oxide layer having an amorphous structure and electroconductivity,a metal layer comprising silver, copper or an alloy thereof, a secondmetal oxide layer having an amorphous structure and electroconductivity,and a third metal oxide layer which has an amorphous structure andcontinuity and which is placed on the upside of the second metal oxidelayer.

The photoelectric conversion layer 22 is a semiconductor layer whichconverts photoenergy of incident light into electric power and therebywhich generates an electric current. The photoelectric conversion layer22 generally comprises a p-type semiconductor sublayer and an n-typeone. The conversion layer can comprise: a laminate structure of p-typepolymer and n-type substance; RNH₃PbX₃ (in which X is a halogen ion andR is an alkyl group or the like); silicon semiconductor; inorganiccompound semiconductor, such as, InGaAs, GaAs, chalcopyrite type, CdTetype, InP type or SiGe type; or quantum dot-containing or dye-sensitizedtransparent semiconductor. Any of them can improve the efficiency andcan reduce deterioration of power output.

For the purpose of promoting or blocking injection of charges, a bufferlayer may be inserted between the conversion layer 22 and thetransparent electrode 21.

The counter electrode 23 is normally an opaque metal electrode, but thetransparent electrode according to the embodiment may be adopted as thecounter electrode 23. Another charge buffer or charge transport layermay be inserted between the counter electrode 23 and the conversionlayer 22.

The buffer or charge transport layer for the anode can be made of, forexample, vanadium oxide, PEDOT/PSS, p-type polymer, vanadium pentoxide(V₂O₅), 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (hereinafter,referred to as “spiro-OMeTAD”), nickel oxide (NiO), tungsten trioxide(WO₃), or molybdenum trioxide (MoO₃).

On the other hand, the buffer or charge transport layer for the cathode,which the transparent electrode serves as, can be made of, for example,lithium fluoride (LIF), calcium (Ca), 6,6′-phenyl-C₆₁-butyric acidmethyl ester (C₆₀-PCBM), 6,6′-phenyl-C₇₁-butyric acid methyl ester(hereinafter, referred to as “C₇₀-PCBM”), indene-C₆₀ bisadduct(hereinafter, referred to as “ICBA”), cesium carbonate (Cs₂CO₃),titanium dioxide (TiO₂),poly[(9,9-bis(3′-(N,N-dimethylamino)propyl-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene))(hereinafter, referred to as “PFN”), bathocuproine (hereinafter,referred to as “BCP”), zirconium oxide (ZrO), zinc oxide (ZnO), orpolyethyleneimine.

Further, a brookite-type titanium oxide layer can be provided betweenthe photoelectric conversion layer and the transparent electrode. It isknown that titanium oxide has three types of crystal structures, namely,rutile, anatase and brookite-type structures. Among them, the titaniumoxide layer of brookite-type structure is preferably adopted in theembodiment. The brookite-type titanium oxide layer easily interacts withthe photoelectric conversion layer, so that excitons generated by lightcan easily undergo charge separation. The brookite-type titanium oxidelayer preferably comprises nanoparticles of brookite-type titaniumoxide. Specifically, the nanoparticles preferably have a mean size of 5to 30 nm. The particle mean size is measured by a particle sizedistribution analyzer. Those nanoparticles of brookite-type titaniumoxide are commercially available, for example, from KOJUNDO CHEMICALLABORATORY CO., LTD.

The counter electrode 23 may have the same structure as the transparentelectrode 21. Further, the counter electrode 23 may compriseunsubstituted planar monolayer graphenes. The unsubstituted monolayergraphene can be formed by a CVD process in which methane, hydrogen andargon are adopted as the reactive gases and a copper foil sheet isemployed as a catalyst undercoat. For example, the monolayer grapheneand a thermal transfer film are pressed and stuck together, and then thecopper undercoat is dissolved to transfer the monolayer graphene ontothe transfer film. The procedure is repeated to stack plural monolayergraphenes on the transfer film. After 2 to 4 graphene layers are thusformed, a metallic connector wiring is printed thereon with silver pasteor the like to produce the counter electrode. The unsubstituted graphenemay be replaced with graphene in which carbons are partly substitutedwith boron atoms. The boron-substituted graphene can be formed from thereactive gases of BH₃, methane, hydrogen and argon in the same manner asdescribed above. Those graphenes can be transferred from the thermaltransfer film onto a proper substrate of, for example, PET.

The mono- or multi-layer graphene can be doped with a tertiary amineserving as an electron donor compound. The electrode comprising theabove graphene layer also functions as a transparent electrode.

On the counter electrode 23, a hole transport layer 25 can be formed.The hole transport layer is, for example, a film ofply(3,4-ethyrenedioxythiophene)/poly(styrenesulfonic acid) compositematerial (PEDOT:PSS). The thickness thereof is, for example, 50 nm.

The solar cell of the embodiment may have a structure sandwiched betweentwo transparent electrodes. The solar cell having that structure canefficiently utilize light incident from both sides. The energyconversion efficiency thereof is generally 5% or more.Characteristically, the solar cell is flexible and can work stably in along term.

The counter electrode 23 may be an ITO glass transparent electrode inplace of the graphene film. In that case, the solar cell losesflexibility but can utilize light energy very efficiently. Further, thecounter electrode may be made of metal, such as, stainless steel,copper, titanium, nickel, chromium, tungsten, gold, silver, molybdenum,tin, zinc, or aluminum. However, that metal electrode tends to beinferior in transparency.

The solar cell according to the embodiment can be also employed as aphotosensor.

Embodiment 2-2

The structure of another photoelectric conversion device according tothe second embodiment is explained below with reference to FIG. 3. FIG.3 schematically shows the structure of an organic EL device(photoelectric conversion device) 30 according to the embodiment. Theorganic EL device 30 has a function of converting inputted electricenergy into light hv. The organic EL device 30 comprises a transparentelectrode 31, a photoelectric conversion layer (luminous layer) 32formed on a surface of the transparent electrode 31, and a counterelectrode 33 formed on the surface of the photoelectric conversion layer32 on the side opposite to the transparent electrode 31.

The transparent electrode 31 is the same as that described inEmbodiment 1. The photoelectric conversion layer 32 is an organic thinlayer in which charges injected from the transparent electrode 31 andfrom the counter electrode 33 are recombined to convert the electricenergy into light. The photoelectric conversion layer 32 normallycomprises a p-type semiconductor sublayer and an n-type one. For thepurpose of promoting or blocking injection of charges, a buffer layermay be inserted between the conversion layer 32 and the counterelectrode 33. Further, another buffer layer may be inserted between theconversion layer 32 and the transparent electrode 31. The counterelectrode 33 is normally a metal electrode but may be a transparentelectrode.

Embodiment 2-3

The structure of still another device according to the second embodimentis explained below with reference to FIG. 4. FIG. 4 schematically showsthe structure of a liquid crystal device 40 according to the embodiment.The liquid crystal device 40 serves as an optical switch in whichorientation of liquid crystal molecules is controlled by application ofvoltage. The liquid crystal device 40 comprises a transparent electrode41, a liquid crystal layer 42 formed on a surface of the transparentelectrode 41, and a counter electrode 43 formed on the surface of theliquid crystal layer 42 on the side opposite to the transparentelectrode 41.

The transparent electrode 41 is the same as that described inEmbodiment 1. The liquid crystal layer 42 is preferably made ofliquid-crystal microcapsules, which are polymer microcapsules enclosingliquid crystal molecules. The layer of liquid-crystal microcapsules canbe formed by wet-coating and drying, and thereafter a metal electrodecan be formed thereon by vapor deposition or sputtering.

Embodiment 3

FIGS. 5A-5E schematically illustrate a method of the embodiment formanufacturing a transparent electrode and a device employing theelectrode.

First, a first metal oxide layer, a metal layer and a second metal oxidelayer are stacked to prepare a laminate structure (Step (I)). There areno particular restrictions on the formation process of the laminatestructure. For example, it can be produced in the following manner.

At the beginning, a first metal oxide layer 12 is formed on a substrate11 (FIG. 5A). This metal oxide layer is produced by, for example,sputtering at a low temperature.

Then, a metal layer 13 is formed on the surface of the metal oxide layer(FIG. 5B). This metal layer is produced by, for example, sputtering orvapor deposition.

Further, a second metal oxide layer 14 is formed on the surface of themetal layer (FIG. 5C). This metal layer is also produced by, forexample, sputtering at a low temperature.

When the first or second metal oxide layer is produced by sputtering,the sputtering temperature is generally room temperature to 150° C. Ifthe temperature is too high, the formed metal oxide layer tends to beless amorphous. It is necessary to pay attention to that. On the otherhand, the metal layer can be produced by sputtering at a hightemperature. In that case, however, the first metal oxide layer isinevitably exposed to high temperature. It is also necessary to payattention to that.

Thereafter, a third metal oxide layer 15 is formed on the surface of thesecond metal oxide layer 14 in the prepared laminate structure. Thethird metal oxide layer 15 is produced by coating the second metal oxidelayer with a metal alkoxide solution and then heating to dry the appliedsolution (Step (II)).

This step is explained below in detail. The metal alkoxide solution isprepared by dissolving an alkoxide of the metal constituting the thirdmetal oxide layer in water or an organic solvent. The solution is thenapplied by a desired method, such as, spin coating, spray coating, orbar coating, to form a metal alkoxide solution layer 15 a on the surfaceof the second metal oxide layer (FIG. 5C).

Subsequently, the metal alkoxide solution layer 15 a is dried to partlyor completely remove the solvent contained therein (FIG. 5D). Thisdrying procedure can be carried out under a reduced pressure, ifnecessary.

Finally, the coating film of the metal alkoxide solution is heated in awater-containing atmosphere, to form a third metal oxide layer 15 (FIG.5E). The heating temperature is 150° C. or less, preferably 120° C. orless, so as to prevent crystallization of the first and second metaloxide layers. The heating procedure is carried out in a water-containingatmosphere so as to accelerate the conversion of metal alkoxide intometal oxide. The atmosphere has a humidity of preferably 30% RH or more,further preferably 40% RH or more.

The transparent electrode according to the embodiment can be produced inthe manner described above. In addition to the above steps, a graphenelayer can be formed on the surface of the second metal oxide layerbefore the step (II) or on the surface of the third metal oxide layerafter the step (II). In that case, the above method preferably furthercomprises the step in which a solution of graphene oxide is applied andthen reduced with hydrazine hydrate.

The transparent electrode thus produced can be used to manufacturevarious devices. For example, on the surface of the third metal oxidelayer in the transparent electrode, a device active part is formeddirectly or via another layer. Subsequently, a counter electrode isfurther formed on the surface of the active part directly or via anotherlayer. The device active part has a structure selected freely accordingto the aimed use, and there are no particular restrictions on theformation method thereof. The device active part can be formed in anyknown manner. However, also when the active part is formed, it isnecessary to pay attention not to crystallize the metal oxide layers ata high temperature.

The counter electrode can be freely selected according to the aimed use.If the whole device needs to be transparent, the counter electrode maybe the transparent electrode of the embodiment. In that case, the thirdmetal oxide layer in the counter electrode is preferably placed on theactive part side. If not required to be transparent, the counterelectrode may be less transparent.

The counter electrode may be a thin metal film. If the counter electrodeis a thin metal film, the thin metal film may be either directly formedon the device active part or independently formed and then stuckthereon.

Further, it is also possible to stack thereon a PET film provided with aUV-cut layer.

EXAMPLES

The embodiments are further explained by the following examples.

Example 1

A transparent electrode 10 having the structure shown in FIG. 1 isproduced. An ITO electroconductive layer 61 is formed by sputtering on a100-μm thick PET film 11. The ITO electroconductive layer has a laminatestructure of a-ITO 12 (thickness: 45 to 52 nm)/silver-Pd alloy 13(thickness: 5 to 8 nm)/a-ITO 14 (thickness: 45 to 52 nm). The surfaceresistance is 7 to 10Ω/square. The laminate structure is thenspin-coated with an isopropanol solution containing titanium (IV)isopropoxide and niobium (V) butoxide. The content of niobium (V)butoxide is 5 wt % to that of titanium (IV) isopropoxide. The appliedsolution is dried in nitrogen at room temperature, and then dried on ahot-plate at 130° C. in an atmosphere having a humidity of 40% toproduce a Nb-doped TiO_(x) layer 15 (thickness: 24 nm). FIG. 6 shows asectional SEM image of the obtained transparent electrode. The TiO_(x)layer is amorphous and continuous. In the image, the number 61represents a thin metal layer for sectional SEM measurement. Tensectional SEM images at different places are observed, and all of themshow that the layer is continuous.

The transparent electrode is left in air at 50° C. for 100 hours, andthen evaluated by XPS. As a result, Ag is not detected on the TiO_(x)surface.

Comparative Example 1

The procedure of Example 1 is repeated except for not spin-coating withthe isopropanol solution containing titanium (IV) isopropoxide andniobium (V) butoxide in such amounts that the content of niobium (V)butoxide is 5 wt % to that of titanium (IV) isopropoxide; to produce atransparent electrode.

The transparent electrode is left in air at 50° C. for 100 hours. Theresultant electrode shows significantly increased resistance. Further,particles of silver oxide are observed on the surface.

Comparative example 2

The procedure of Example 1 is repeated except for spin-coating not withthe isopropanol solution containing titanium (IV) isopropoxide andniobium (V) butoxide in such amounts that the content of niobium (V)butoxide is 5 wt % to that of titanium (IV) isopropoxide, but with anethanol dispersion of crystalline titanium oxide nanoparticles; toproduce a transparent electrode.

The transparent electrode is left in air at 50° C. for 20 hours, andthen evaluated by XPS. As a result, Ag is detected on the TiO_(x) layersurface, and this indicates that migration occurs.

Comparative example 3

The procedure of Example 1 is repeated except that the applied solutionis not dried in nitrogen at room temperature and then on a hot-plate at130° C. in an atmosphere having a humidity of 40%, but dried on ahot-plate at 130° C. in an atmosphere having a dew point of −10° C.; toproduce a transparent electrode.

The transparent electrode is left in air at 50° C. for 20 hours, andthen evaluated by XPS. As a result, Ag is detected on the TIO_(x) layersurface, and this indicates that migration occurs.

Example 2

An electroconductive layer is formed by sputtering on a 100-μm thick PETfilm. The electroconductive layer has a laminate structure of a-AZO(thickness: 40 to 50 nm)/silver (thickness: 5 to 8 nm)/a-ITO (thickness:50 to 55 nm). The surface resistance is 10 to 12Ω/square. The laminatestructure is then spin-coated with an isopropanol solution containingmolybdenum (VI) isopropoxide. The applied solution is dried in nitrogenat room temperature, and then dried on a hot-plate at 130° C. in anatmosphere having a humidity of 40% to produce a MoO_(x) layer(thickness: 20 to 25 nm). A sectional SEM image of the obtainedtransparent electrode indicates that the MoO_(x) layer is amorphous andcontinuous.

The transparent electrode is left in air at 50° C. for 100 hours, andthen evaluated by XPS. As a result, Ag is not detected on the MoO_(x)surface.

Example 3

An electroconductive layer is formed by sputtering on a 100-μm thick PETfilm. The electroconductive layer has a laminate structure of a-AZO(thickness: 40 to 50 nm)/silver (thickness: 5 to 8 nm)/a-ITO (thickness:50 to 55 nm). The surface resistance is 10 to 12Ω/square. The laminatestructure is then spin-coated with an isopropanol solution containingnickel isopropoxide. The applied solution is dried in nitrogen at roomtemperature, and then dried on a hot-plate at 130° C. in an atmospherehaving a humidity of 40% to produce a NiO_(x) layer (thickness: 20 to 25nm). A sectional SEM image of the obtained transparent electrodeindicates that the NiO_(x) layer is amorphous and continuous.

The transparent electrode is left in air at 50° C. for 100 hours, andthen evaluated by XPS. As a result, Ag is not detected on the NiO_(X)surface.

Example 4

An electroconductive layer is formed by sputtering on a 100-μm thick PETfilm. The electroconductive layer has a laminate structure of a-ITO(thickness: 50 to 55 nm)/copper (thickness: 5 to 8 nm)/a-ITO (thickness:50 to 55 nm). The surface resistance is 15 to 20Ω/square. The laminatestructure is then spin-coated with an isopropanol solution containingtitanium (IV) isopropoxide. The applied solution is dried in nitrogen atroom temperature, and then dried on a hot-plate at 130° C. in anatmosphere having a humidity of 40% to produce a TiO_(x) layer. Asection of the obtained transparent electrode indicates that the TiO_(x)layer is amorphous and continuous.

The transparent electrode is left in air at 50° C. for 300 hours, andthen evaluated by XPS. As a result, copper is not detected on theTiO_(x) surface.

Example 5

An electroconductive layer is formed by sputtering on a PET film havinga thickness of 100 μm. The electroconductive layer has a laminatestructure of a-ITO/Ag—Pd alloy/a-ITO. The surface resistance is 5 to8Ω/square. The laminate structure is then spin-coated with an ethanolsolution containing tetraethoxysilane (TEOS). The applied solution isdried in nitrogen at room temperature, and then dried on a hot-plate at130° C. in an atmosphere having a humidity of 40% to produce a SiO_(x)layer. A section of the obtained transparent electrode shows that theSiO_(x), layer is amorphous and continuous.

The transparent electrode is left in air at 50° C. for 100 hours, andthen evaluated by XPS. As a result, Ag is not detected on the SiO_(x)surface.

Example 6

An electroconductive layer is formed by sputtering on a PET film havinga thickness of 100 μm. The electroconductive layer has a laminatestructure (thickness: 120 nm) of a-ITO/Ag—Pd alloy/a-ITO. The surfaceresistance is 5 to 8Ω/square. The laminate structure is then spin-coatedwith an ethanol solution containing isopropoxy aluminum. The appliedsolution is dried in nitrogen at room temperature, and then dried on ahot-plate at 130° C. in an atmosphere having a humidity of 40% toproduce a AlO_(x) layer. A section of the obtained transparent electrodeshows that the AIO_(x) layer is amorphous and continuous.

The transparent electrode is left in air at 50° C. for 100 hours, andthen evaluated by XPS. As a result, Ag is not detected on the AlO_(x)surface.

Example 7

A transparent electrode 70 having the structure shown in FIG. 7 isproduced. An electroconductive layer 72 is formed by sputtering on a100-μm thick PET film 71 in the same manner as in Example 1. Theelectroconductive layer has a laminate structure of a-ITO/silver-Pdalloy/a-ITO. The surface resistance is 8 to 10Ω/square. The surface isthen coated with a shielding layer in which planar N-graphene films arestacked in four layers on average. Each graphene film comprises carbonatoms partly substituted with nitrogen atoms.

The shielding layer is formed in the following manner. At the beginning,a sheet of Cu-foil is irradiated to heat with laser beams and thenannealed to grow crystallites.

The Cu-foil sheet thus surface-treated is adopted as a catalystundercoat, on which a planar monolayer N-graphene film is formed from areactive gas mixture of ammonia, methane, hydrogen and argon(15:60:65:200 ccm) by CVD method at 1000° C. for 5 minutes. In mostcases, a monolayer N-graphene film is formed as a result of that CVDprocess. However, depending on the conditions, a two or more-layerN-graphene film is partly formed. Subsequently, the formed N-graphenefilm is further treated in an ammonia-argon mixed gas stream at 1000° C.for 5 minutes, and then cooled in an argon stream. The obtainedmonolayer N-graphene film and a thermal transfer film are pressed andstuck together, and then they are immersed in copper(II) chlorideetchant, so that the copper undercoat is dissolved to transfer themonolayer N-graphene film onto the transfer film. Those procedures arerepeated to stack four monolayer N-graphene films on the thermaltransfer film. In this way, a multilayer graphene film is produced.

Thereafter, the thermal transfer film is laminated on a structure ofa-ITO/silver-Pd alloy/a-ITO/PET film, and then heated to transfer theN-graphene film onto the structure of a-ITO/silver-Pd alloy/a-ITO/PETfilm to form a shielding layer 73.

The nitrogen content in the graphene film is measured by XPS, and foundto be 1 to 2 atm % under the above conditions. The atomic ratio betweencarbon and oxygen is also measured by XPS, and found to be 100 to 200.

Subsequently, in the same manner as in Example 1, the shielding layer iscoated with an isopropanol solution containing titanium (IV)isopropoxide and niobium (V) butoxide in such amounts that the contentof niobium (V) butoxide is 5 wt % to that of titanium (IV) isopropoxide.The applied solution is dried in nitrogen at room temperature, and thendried on a hot-plate at 130° C. in an atmosphere having a humidity of40% to produce a Nb-doped titanium oxide layer 74.

Meanwhile, the PET film surface on the side opposite to theelectroconductive layer is screen-printed with UV-cut ink containing2-hydroxy-4-methoxybenzophenone, to form a UV-cut layer 75. Finally, asilica film is formed by vacuum deposition to produce a gas-barrierlayer 76 on the UV-cut layer. Thus, a transparent electrode 70 isobtained.

The transparent electrode is exposed from the gas-barrier layer side tovisible light of 10 suns for 100 hours at a temperature of 80° C. and ahumidity of 80%, and then evaluated by XPS. As a result, Ag is detectedneither on the TiO_(x) surface nor on the surface of the gas-barrierlayer.

Example 8

A solar cell shown in FIG. 8 is produced.

The transparent electrode 81 obtained in Example 1 is coated with atoluene solution of C60-PCBM by means of a bar-coater. The appliedsolution is dried to form an electron transport layer 82. Further, thelayer is coated with a chlorobenzene solution containing C60-PCBM andpoly(3-hexylthiophene-2,5-diyl) (hereinafter, referred to as “P3HT”) bymeans of a bar-coater. The applied solution is dried at 100° C. for 20minutes, to form a photoelectric conversion layer 83.

Meanwhile, a sheet of stainless steel-foil provided with an insulatingceramic film on one surface is prepared, and the other surface thereofis treated with dilute hydrochloric acid to remove a surface oxidizedfilm. The surface thus treated was then coated with an aqueous solutionof graphene oxide by means of a bar-coater, to form a graphene oxidelayer. After dried at 90° C. for 20 minutes, the graphene oxide layer istreated with hydrazine hydrate vapor at 110° C. for 1 hour to covertinto a shielding layer 85. The shielding layer is a two-layer N-graphenefilm in which carbon atoms of the graphene oxide are partly substitutedwith nitrogen atoms.

The N-graphene film 85 is coated with an aqueous solution of PEDOT.PSScontaining sorbitol by means of a bar-coater. The applied solution isdried at 100° C. for 30 minutes to form an adhesive layer 86 (thickness:50 nm) containing PEDOT.PSS.

The obtained sheet is then so laminated at 90° C. on the photoelectricconversion layer 83 that the adhesive layer 86 may be in contacted withthe conversion layer.

Subsequently, the PET film surface on the side opposite to theelectroconductive layer is screen-printed with UV-cut ink containing2-hydroxy-4-methoxybenzophenone, to form a UV-cut layer 87. Finally, asilica film is formed by vacuum deposition to produce a gas-barrierlayer 88 on the UV-cut layer. Thus, a solar cell 80 is obtained.

The obtained solar cell converts sunlight of 1 sun at an efficiency of5% or more. Even if the cell is left in the open air for a month, theefficiency deteriorates only by less than 2%.

Example 9

An organic EL device 90 shown in FIG. 9 is produced.

On the titanium oxide surface of the transparent electrode 91 obtainedin Example 1, a photoelectric conversion layer 92 serving as a luminouslayer (40 nm) is formed by vapor-deposition oftris(8-hydroxyquinolinato)aluminum (Alq₃), which functions as an N-typesemiconductor. Further,N,N′-di-1-naphthyl-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine(hereinafter, referred to as “NPD”) is vapor-deposited thereon to form ahole transport layer 93 of 30 nm thickness.

Furthermore, a metal electrode 94 is formed thereon by sputtering.Finally, the circumference is shielded to produce an organic EL device.

The obtained organic EL device hardly deteriorates in output light. Evenafter continuous operation for 1000 hours, the output decreases only by7% or less.

Example 10

A transparent organic EL device is produced.

On the titanium oxide surface of the transparent electrode (negativeelectrode) obtained in Example 1, a photoelectric conversion layerserving as a luminous layer (40 nm) is formed by vapor-deposition oftris(8-hydroxyquinolinato) aluminum (Alq₃), which functions as an n-typesemiconductor. Further,N,N′-di-1-naphthyl-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine(hereinafter, referred to as “NPD”) is vapor-deposited thereon to form ahole transport layer of 30 nm thickness.

The hole transport layer is then coated with a methanol solution ofnickel acetate by means of a bar-coater. The applied solution is driedat 130° C. for 10 minutes to form an amorphous and continuous NiO_(x)layer (thickness: 20 nm). On the formed NiO_(x) layer, a structure ofa-ITO (40 nm)/silver (10 nm)/a-ITO (50 nm) is formed by sputtering toproduce a transparent electrode (positive electrode). Finally, thecircumference is shielded to produce a transparent organic EL device.

The obtained organic EL device hardly deteriorates in output light. Evenafter continuous operation for 1000 hours, the output decreases only by7% or less.

Example 11

A liquid crystal cell 100 shown in FIG. 10 is produced.

A transparent electrode 101 is formed in the same manner as that inExample 6. The AlO_(x) surface of the electrode is then coated with anaqueous dispersion of liquid crystal microcapsules. The applieddispersion is dried to form a liquid crystal microcapsule layer 102.Subsequently, the microcapsule layer is coated with an aqueousdispersion of PEDOT.PSS containing sorbitol. The applied dispersion isthen dried to form an electroconductive adhesive layer 103. Meanwhile, atransparent electrode 104 is produced in the same manner as that inExample 4. The transparent electrode 104 is so laminated on the adhesivelayer that the SiO_(x) surface of the electrode may be in contacted withthe adhesive layer. Finally, the circumference is shielded to produce aliquid crystal cell. This device is opaque, but becomes transparentunder application of voltage. Accordingly, it can serve as an opticalswitch.

The obtained liquid crystal cell hardly deteriorates in drive voltagelight. Even after continuous operation for 1000 hours, the voltageincreases only by 5% or less.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fail within thescope and sprit of the invention.

1. A transparent electrode having a laminate structure comprising: afirst metal oxide layer having an amorphous structure andelectroconductivity, a metal layer which comprises a metallic materialcomprising silver or copper, a second metal oxide layer having anamorphous structure and electroconductivity, and a third metal oxidelayer having an amorphous structure and continuity, stacked in thisorder.
 2. The transparent electrode according to claim 1, wherein one orboth of the first metal oxide layer and the second metal oxide layercomprises a substance selected from the group consisting of indium-dopedtin oxide, aluminum-doped zinc oxide, and fluorine-doped tin oxide. 3.The transparent electrode according to claim 1, wherein the third metaloxide layer comprises a substance selected from the group consisting oftitanium oxide, tin oxide, zinc oxide, and zirconium oxide.
 4. Thetransparent electrode according to claim 1, wherein the third metaloxide layer comprises a substance selected from the group consisting ofcopper oxide, nickel oxide, molybdenum oxide, iron oxide and tungstenoxide.
 5. The transparent electrode according to claim 1, wherein thethird metal oxide layer comprises a substance selected from the groupconsisting of silicon oxide and aluminum oxide.
 6. The transparentelectrode according to claim 1, wherein the first metal oxide layer andthe second metal oxide layer comprise an indium-doped tin oxide, and themetal layer comprises a metallic material comprising silver.
 7. Thetransparent electrode according to claim 1, further comprising agraphene layer in direct contact with the third metal oxide layer. 8.The transparent electrode according to claim 7, wherein the graphenelayer is doped with nitrogen atoms.
 9. The transparent electrodeaccording to claim 1, wherein the third metal oxide layer is doped withother metal ions having a valence different from that of the metalconstituting the metal oxide.
 10. The transparent electrode according toclaim 1, wherein the third metal oxide layer is thinner than the secondmetal oxide layer.
 11. The transparent electrode according to claim 1,wherein the third metal oxide layer comprises carbon or nitrogen. 12.The transparent electrode according to claim 1, wherein the third metaloxide layer comprises alkoxy-metal bonds.
 13. An electronic devicecomprising the transparent electrode according to claim 1 and a counterelectrode.
 14. The electronic device according to claim 13, furthercomprising a UV-cut layer or a gas-barrier layer.
 15. The electronicdevice according to claim 13, wherein the counter electrode is a secondtransparent electrode.
 16. A method for manufacturing a transparentelectrode, comprising: (I) preparing a laminate structure comprising: afirst metal oxide layer having an amorphous structure andelectroconductivity, a metal layer which comprises a metallic materialcomprising silver or copper, and a second metal oxide layer having anamorphous structure and electroconductivity; and thereafter (II) coatingthe surface of the second metal oxide layer with a metal alkoxidesolution, and then drying and heating the applied solution at 150° C. orless in a water-containing atmosphere, to form a third metal oxidelayer.
 17. The method according to claim 16, further comprising forminga graphene layer before or after (II).
 18. The method according to claim17, wherein the forming a graphene layer comprises: applying a solutionof graphene oxide and then reducing the applied solution with hydrazinehydrate.